On a map, Boston is some 3,000 miles from Marin, but in our hearts we are next-door neighbors, sharing our shock, grief and courageous resolve in response to the bombings at the Boston Marathon.

The unthinkable tragedy was yet another reminder that it could happen here, or anywhere in America.

Flags were lowered to half-staff in honor of the dead as we, as a nation, once again mourned. Many paused for a moment of silence to consider the loss of three innocent spectators and injuries to nearly 200 more.

Monday's tragedy was a jolting reminder of the precarious times in which we live.

A storied American running race that celebrates human spirit and endurance quickly became a race to save lives. It also became reason to strengthen our resolve to search for answers and an end to such senseless deaths.

Officials say they are still searching for a cause, some sort of explanation why someone would plant two bombs at a popular event with a deranged intent to murder and maim innocent people.

We, as a nation, are searching for an explanation for an unthinkable act of terror.

There is no other word to describe the intent of whomever packed bombs with ball bearings and nails to be detonated in the middle of a crowd.

President Barack Obama has been careful to not connect the horrific attack to our military campaigns in Iraq or Afghanistan, even though such bombings are commonplace in those strife-torn nations.

We shouldn't forget that we don't need misguided outside forces to engage in acts of terror. We, too, have grown too many of our own.

Oklahoma City, Newtown, Columbine and Aurora come to mind. There are too many others, as well.

Monday's blasts claimed the life of 8-year-old Martin Richard. His mother and sister were among those seriously injured.

Two brothers, who had come to watch the finish, each lost a leg in the blast.

President Obama has not ruled out that the bombings could be the work of a "malevolent individual."

Security measures have been stepped up across the country. That's appropriate. Security measures were in place for the Boston Marathon, but none are absolutely fail safe.

The Marin Board of Supervisors paused Tuesday for a moment of silence. Supervisor Kate Sears urged that we need to restore our focus on "the common good."

Events such as Monday's tragedy certainly shake our national sense of safety and security, but not our resolve that courage and human compassion are our strongest weapons against hate and intolerance.

The strength of that resolve melts away those miles between Boston and Belvedere.